1. Field of the Invention
The present invention relates to nanotube manufacturing, and more specifically, to nanotubes and systems and methods for the formation and/or manufacture of nanotubes and nanotube structures.
2. Related Art
Carbon nanotubes are tubular carbonaceous structures with mechanical, electrical, and chemical properties that make them potentially useful in many fields, including electronic, mechanical, and medical applications. For example, they exhibit exceptional strength, primarily due to the presence of strong sp2 bonds between the carbon atoms making up the tubes. Furthermore, they exhibit interesting electrical properties, such as the high conductivity of some tubes due to the alignment of carbon atoms along the long axis of the tubes. The likewise exhibit thermal properties that make them attractive for various uses, such as in heat sinks for computer chips. In addition, because they are hollow, they can hold, transport, and ultimately release substances. This property makes them quite useful for medical applications. Numerous studies are being conducted to identify other unique and useful properties of these small structures.
A nanotube is a cylindrical carbon lattice having a basic lattice structure of a fullerene. Most nanotubes are capped at one or both ends by a half fullerene molecule. Nanotubes are characterized by having external diameters of one nanometer (1 nm) to only a few (e.g., 5-10) or tens (e.g., 50) nanometers. While many nanotubes are only a few times longer than they are wide, some have been fabricated having a length of millions of times greater than their width. Nanotubes can align themselves into rope-like structures, permitting fabrication of long wires of exceptional strength, yet relatively light weight.
Nanotubes have been fabricated in two different types of basic structures: single-walled nanotubes (SWNT), and multi-walled nanotubes (MWNT). As their names imply, SWNTs are tubes having a single wall encasing an internal volume, whereas MWNTs are tubes in which a single internal volume is encased by multiple tubular wall structures arranged as nested cylinders. Due to their different structures, and due to the differences in the ease by which they can be produced, SWNTs and MWNTs are being targeted and used for different purposes (although many uses overlap).
Currently, there are various known processes and methods for the production or manufacture of carbon nanotubes. These processes can include Arc Discharge, Laser Ablation, and Chemical Vapor Deposition. In the Arc Discharge method, a carbon-containing vapor is created by an arc discharge between two carbon electrodes, and carbon nanotubes self-assemble from the vapor. Unfortunately, this method results in high levels of impurities that are expensive to remove, if at all possible. In the Laser Ablation method, a high-energy laser beam impinges on a volume of carbon-containing feedstock gas. While the nanotubes produced by Laser Ablation are cleaner than those produced by Arc Discharge, the yield is significantly lower. In the Chemical Vapor Deposition method, carbon-containing gas is exposed to heated reactive metal, which causes formation of nanotubes on the heated surface of the metal. Chemical Vapor Deposition can be used on a large scale, but often and uncontrollably produces a mixture of SWNTs and MWNTs having a wide range of diameters, the SWNTs invariably being of poor quality. Furthermore, it requires purification of the nanotubes from the soot and metals present in the reaction.
U.S. Pat. No. 6,455,021 discloses of an arc discharge method, whereby a flow of a precursor gas is exposed to a plasma discharge at very high temperatures in the production carbon nanotubes. The nanotubes generated through this protocol, however, can include a good volume of contaminants.
U.S. Pat. No. 6,331,690 discloses a laser ablation method in connection with the production of nanotubes, whereby a high-energy laser is focused at a carbon target. This method can produce nanotubes with relatively fewer contaminants than the arc discharge method, but the production rate can be low. The laser ablation method can also be capital-intensive.
U.S. Pat. No. 6,689,674 discloses of a Chemical Vapor Deposition (CVD) method for the production of nanotubes, whereby a flow of precursor gas is heated and directed over a reactive metal surface. The use of CVD in the production of carbon nanotubes can generate a good yield and relatively fewer contaminants. However, the carbon nanotubes produced can have a number of defects.
Due to the complexity of the fullerene lattice and the various ways it can be wrapped to form a cylinder or tube, nanotubes having different lattice conformations can have different physical properties. Three main classifications of nanotube lattices are uesd: zig-zag, chiral, and armchair. In general, the differences between these three classifications can be thought of as based on the orientation of a graphine sheet, before being wrapped into a tube, relative to a central axis along the tube.
These presently available nanotube-manufacturing methodologies, as noted, can result in nanotubes with a spectrum of variability in their physical properties, including number of walls, length, diameter, and lattice structure. Thus, the current technologies do not permit one to pre-select and produce only one type of nanotube, having a single wall type, length, diameter, and lattice structure or conformation. The manufacturing cost associated with such high temperature growth processes is high due to the energy cost and time required with such batch type processes.
Thus, there is a need for a reliable, consistent, controlled, and cost effective approach, so that nanotube structures may be generated within a mass production process with specificity as to length, diameter, and lattice structure, among other things.